An investigation into concentration and rate of reaction

An investigation into concentration and rate of reaction
An investigation into concentration and rate of reaction Aim: to determine the effect on rate of reaction of changing the concentration of sodium thiosulphate in the reaction sodium thiosulphate + Hydrochloric acid → Sodium Chloride + sulphur dioxide + Sulphur + water. Scientific background A reaction is a change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances. The particular reaction of sodium thiosulphate + Hydrochloric acid → Sodium Chloride + sulphur dioxide + Sulphur + water or Na2S2O3 + 2HCl → 2NaCl + SO2 + S + H2O. This is a precipitate reaction in which a precipitate (in this case sulphur) is formed, because of this we can tell when the reaction is complete by judging when a certain amount of sulphur is produced.
The collision theory states that for a reaction to take place particles have to ?bump? into each other with sufficient force. This theory helps to explain what effects certain conditions will have on the rate of reaction. For example, a higher temperature means that the particles will be moving more quickly and with more energy so are more likely to ?bump? into each other and cause a reaction. So a higher temperature will normally speed the rate of a reaction. Other factors that change the rate of reaction are whether a catalyst is present (gives a surface for them to react on) and the size of particles (increased surface area means a quicker reaction). This investigation will be into concentration. The collision theory means that a higher concentration (or pressure for gases) of a reactant will mean a faster reaction because the particles will be pressed close together. This is shown in the diagrams here: Prediction: The higher the concentration of sodium thiosulphate used, the faster the reaction will be. The collision theory means that a higher concentration will give a faster rate of reaction, as the particles of sodium thiosulphate will be more likely to ?bump? into the particles of HCl. Equipment Conical flask 2 beakers 2 measuring cylinders White paper with a cross drawn on in black pen Stopwatch Goggles Method We will place the conical flask over the paper with the cross on it directly over the midpoint of the cross. Then we will collect a beaker full of acid and a beaker full of sodium thiosulphate. We will then measure out 10cm3 of acid into a measuring cylinder and pour it into the conical flask. Then we will measure out 10cm3 of sodium thiosulphate into the other measuring cylinder before pouring that into the conical flask. As soon as the sodium thiosulphate touches the acid we will start the stopwatch. We will have one person swirling the conical flask, one person timing how long it takes to react and one person to decide when the cross can no longer be seen. We will stop timing when the cross is no longer visible because it means that a certain amount of sodium has been produced and that the reaction has got to a certain point. Then we will take down the amount of time it took. Then we will wash out the conical flask, repeat the experiment five times with the same concentration of sodium thiosulphate. After that we will try with different concentrations. Our preliminary work indicated that good concentrations to try would be 10g/l, 20g/l, 30g/l, 35g/l, 40g/l, 45g/l and 50g/l because they will give varied results and accurate results. Under 10g/l will take too long to react and 15 and 25 will not be included because of time constraints. 10cm3 was decided to be a good volume of reactants to use, even though the reaction was not completed at 10g/l, because the amount of sodium thiosulphate available will not really be enough to complete the experiment if more is used. 35g/l and 45g/l of sodium thiosulphate are not available to use as they are so they will be made by mixing 5cm3 of two other concentrations to get those. Fairness in testing It is important that the same people are used to do each thing every time to ensure there are no differences and that it will be a fair test. The same concentration of hydrochloric acid must be used and the same size conical flask used. The only variable to be changed should be the concentration of sodium thiosulphate. However, the temperature cannot be controlled so the temperature at the time of each test should be recorded and shown if there are any strange results this will probably be the cause. Safety Acid is being used so care should be taken to avoid spillages. Goggles should also be worn to prevent harmful substances entering the eyes. Acid and sodium thiosulphate should be poured into beakers before being measured due to the thin rims of the measuring cylinders so as to avoid spillages. No one should eat or drink while the experiment is going on in case of contamination. Results Concentration (g/l) 1 2 3 4 5 Average 10 Over 1200 Over 1200 Over 1200 Over 1200 Over 1200 Over 1200 20 127.62 130.63 129.56 129.23 128.66 129.14 30 98.78 98.38 97.5 98.22 97.76 98.128 35 68.01 66.53 69.3 67.45 71.2 68.498 40 66.81 59.22 65.37 61.16 60.97 62.706 45 52.1 50.28 53.4 52.86 51.52 52.032 50 39.4 38.6 42.85 42.81 38.21 40.374 Concentration (g/l) 1/Average 10 0.00083 20 0.0077 30 0.01 35 0.014 40 0.015 45 0.019 50 0.024 Conclusion The results show that as the concentration of sodium thiosulphate increases the rate of reaction becomes faster. This is as I predicted and ties in with the collision theory. This is because there is more sodium thiosulphate in the same size space and so is more likely to meet with the acid. This is explained in the scientific background part but the collision theory is that the molecules have to meet with enough energy to react. By having more sodium thiosulphate molecules in the same space it meant they were more likely to meet with enough energy and so the reaction was faster with a higher concentration The 10g/l concentration had so little sodium thiosulphate molecules in it that there were not enough there to form enough sulphur that the cross could no longer be seen. This is why the graph should be a curved line for the average times. The 1 divided by time graph shows that there is a strong positive correlation between them and that is almost a straight line, as it should reach zero when there is no reaction as there is no sodium thiosulphate. Evaluation There were several problems with the experiment. The temperature was different on different days and that caused the 35g/l test results to be quicker, as they were taken on a day that was 50 higher than the others. It was at 29 degrees whereas the 40g/l was at 24 degrees. The 10g/l did not reach a point where enough sulphur had been formed to hide the cross. It would be better to use 15g/l as the lowest concentration to give the results a better depth and show the true shape that the graph should be. Ideally a greater range of concentrations would also have been used. A good range would have been from 15 to 80 doing every 5g/l five times. This was not possible this time due to time constraints and the lack of sodium thiosulphate at a concentration higher than 50g/l There was too much scope for human error in the way the experiment was conducted. By having a person swirling the conical flask it allowed differences between the strength of the swirling from each experiment. It would have been better to put it on something with a steady rotation. It was up to a single person to decide when the cross was no longer visible. A better way to do that would be to use a computer with colour recognition that could tell when the cross was no longer visible or ideally to have a computer that can identify how many molecules of sulphur had been formed. A computer would have been better to time with as well because human reaction times are slow in comparison. Another experiment to further the results of this would be to vary the concentration of acid instead of sodium thiosulphate to prove conclusively that it is not just a one-off case. A reaction involving a gas and pressure such as the reaction between steam and iron would also be good to use as it shows this is not just for solids and liquids. The reaction between steam and iron would have to be measured n a different way though as a precipitate is not formed. The weight could be taken because in the reaction hydrogen is given off and so the weight would decrease and when it reached a certain point, such as 5g of hydrogen given off, then the experiment time could be recorded to see if it matches with the results from this.

An investigation into concentration and rate of reaction 9.9 of 10 on the basis of 3069 Review.